1. Field of the Invention
The present invention is related to a method and an apparatus for detecting a sight line vector of a human. Particularly, the present invention is related to a technique for detecting a sight line direction of a person as, for instance, assisting means for an input unit of a computer or when driving a vehicle.
2. Description of the Related Art
When a person wishes to view a scene or an event, the person turns his or her face and eyes to a direction of the scene or the event. That is, the person turns his or her face and eyes to the direction that the person wishes to see in response to an instruction from a brain, images the desired scene or event on retinas, and then, processes images on the retinas so as to acquire necessary information. In this case, turning the eye means making a normal vector of a pupil of the eye correspond to a desired direction, and it is conceivable that a sight line direction (otherwise, sight line vector) is a half straight line (otherwise, vector) which connects a center point of a cornea ball and a center point of the pupil. As a consequence, in order to detect the sight line direction, the sight line direction is obtained by determining a position (coordinates) of the center point of the cornea ball and a position (coordinates) of the center point of the pupil.
Since a pupil is externally exposed, if a distance from a camera up to the pupil can be measured, then center of the pupil can be easily calculated. For instance, by photographing a pupil, a center of the pupil can be acquired from the photographed image. However, it is difficult to directly measure the distance from the camera up to the pupil. Also, since a center of a cornea ball is not externally exposed, it has been so understood that a calculation of this position is complicated. As a consequence, such a sight line vector detecting method and a sight line vector detecting apparatus which are simple and easily understood have not yet been developed.
In order to acquire the center position of the cornea ball, for instance, it is conceivable that a cornea shape measuring apparatuses for measuring a cornea shape, which is used in a hospital's ophthalmology department, may be utilized. However, as can be easily understood from the techniques described in, for example, JP-A-2003-38442 (cornea shape measuring apparatus) and JP-A-2002-17674 (cornea measuring apparatus), these cornea shape measuring apparatuses are not provided to measure entire shape of the cornea ball, but to measure a surface shape on a front side of the cornea. Therefore, it is difficult to measure the center position of the cornea by these cornea shape measuring apparatuses.
As a method of acquiring a sight line direction, for example, there is one method disclosed by HONDA MOTOR CO., LTD. and GENTECH CORPORATION (US publication No. 2005/0200806 A1, “Line-of-sight detection method and apparatus therefor”). FIG. 9 represents a relationship between a structure of an eyeball and a sight line direction. In FIG. 9, a cornea 55 is located on the outer surface of an eyeball 50, and the cornea 55 is regarded as a portion of a spherical surface of a cornea ball 56 shown by a dotted line. A center of the cornea ball 56 is indicated as S, and the cornea ball 56 is a virtual shell having a radius R. An iris 58 for adjusting an incident light amount is positioned on the front side of a lens 57, and a portion which can be seen from an opening of the iris 58 is a pupil 59. A half straight line 60 of an arrow which connects the center S of the cornea ball 56 and a center T of the pupil 59 corresponds to a sight line direction (sight line vector). It should be noted that FIG. 9 indicates a photographed image 61 and a light beam which is emitted from an LED light source B and is reflected at a point “P” on the cornea 55 to reach a center O of a camera lens. The method of acquiring the sight line direction described in US publication No. 2005/0200806 A1 utilizes the below-mentioned four assumptions (1) to (4) which are recognized based on morphologically known data (“Anthropometry of the Head and Face”, by L. G. Farkas, Lippincott Williams & Wilkins, 1994). That is, in such a case that, on a face of a subject, a right eye is a point A, a left eye is a point B, and a nose is a point C, the below-mentioned assumptions are made:    distance (A, C)=distance (B, C)    ratio {distance (A, B)/distance (A, C)}=1.0833    distance (A, B)=6.5 cm    diameter of cornea ball=1.54 cm (radius=7.7 mm).
There is a certain question whether all of the above-described assumptions are correctly applied to all of persons. As a consequence, it is necessary to perform a test on the subject. Also, calculation for acquiring the sight line direction is complicated.